Not Applicable.
This invention relates in general to antifriction bearings, and more particularly to a system and process for lubricating such bearings.
The spindle of a precision machine tool must rotate with considerable stability; that is to say, it must not deviate from its axis of rotation. Wobble or radial and axial motion in the spindle cannot be tolerated. Angular contact ball bearings and tapered roller bearings make this possible. When the spindle is supported in a housing on a pair of either of these types of antifriction bearings, with the bearings of the pair being mounted in opposition, one of the bearings may be adjusted against the other to bring the bearings into a condition of preload. In that condition no radial or axial clearances exist within the bearings, and the spindle rotates without deviation from the axis of rotation.
In an angular contact ball bearing, the balls roll along raceways that are arcuate in cross section and generally conform to the contour of the balls. But the balls contact the raceway of the outer race in a region offset axially from the region where they contact the raceway of the inner race, so that the raceways are in a sense oblique to the axis of the bearing. This enables the bearing to carry thrust as well as radial loads. In the absence of lubrication along the raceways, scoring and welding of surface finish asperities will occur, not only along the raceways, but on the spherical surfaces of the balls as well.
In a tapered roller bearing the tapered rollers roll along tapered raceways which lie within conical envelopes having their apices at a common point along the axis of rotation. This places the rollers xe2x80x9con apexxe2x80x9d so that pure rolling occurs along the raceways. Hence, little, if any, friction develops between the rollers and the tapered raceways. But the geometry of a tapered roller bearing is such that the rollers will migrate up the raceways and unless restrained will be expelled from the bearing. A thrust rib, at the large end of one of the raceways, provides the restraint. The rollers at their large end faces bear against the thrust rib and, when the bearing is set into operation, those end faces spin and slide along the face of the rib. The spinning and sliding produces friction, and unless a film of lubrication is maintained along the rib face, scoring will develop and the bearing will overheat and perhaps fail.
Standard grease provides adequate lubrication for the spindle bearings of either type, provided they rotate at moderate velocities. It costs little and may be introduced to critical surfaces through a single set of passageways. Moreover, it requires no break in. But standard grease churns at higher speeds as it passes between the rolling elements and the raceways for the bearings, and this requires work which manifests itself in the form of heat. The heat raises the temperature of the bearings. In a set of two or more bearings mounted in opposition, the heat produces temperature differentials between the housing and spindle and upsets the fine tolerances used in the manufacture of the machine tool. Circulating oil accommodates somewhat higher spindle speeds, but it requires an expensive system of pumps, filters and oil passages. Furthermore, oil likewise churns or is worked as it passes between the rolling elements and the raceways, and to dissipate the heat that is produced an oil cooler is sometimes required.
The bearings which operate at still higher speeds require more sophisticated lubrication. One is oil mist. It requires compressed air to operate a mist generator and move the mist through passageways to critical surfaces in the bearings. Compressed air does not come inexpensively. Not only must air be compressed, but it further must be dried and filtered, since it comes in contact with the critical surfaces in the bearings. Apart from that, a high speed bearing must rely on low friction seals to contain the mist, and they are not very effective in this respect. The mist escapes and coats nearby objects with a messy and slippery film. It also pollutes the surrounding atmosphere, producing what some consider to be a health hazard.
An oil-air lubrication system will also satisfy the requirements of high speed spindle bearings. This system relies on compressed air to detach oil in the form of drops from a feed pipe and direct those drops to critical surfaces in the bearing. The compressed air also cools the bearing. Unlike the oil mist system, the oil-air system does not pollute the surrounding atmosphere, for any oil that escapes remains as liquid oil, not a mist. But an oil-air system requires compressed air, which is expensive to produce, and it also requires complex tubing to deliver the oil and air to critical surfaces in the bearing.
High performance greases afford an alternative to oil-mist and oil air systems. This synthetic product, when used sparingly, provides adequate lubrication for high speed spindle bearings with very low heat generation and therefore low operating temperatures. A standard grease or oil when used as a lubricant in an antifriction bearing produces an elastrohydrodynamic (EHD) lubricant film between critical surfaces in the bearing, primarily between the raceways and the rolling elements. Here the EHD film is compressed under enormous pressure. This requires work, and, like friction, the work manifests itself in the form of heat. Indeed, tests on a tapered roller bearing have shown that in high speed operation, the working of the EHD film along the raceways consumes more energy than the friction produced along the face of the thrust rib. High performance greases produce a thinner film, sometimes referred to as xe2x80x9cboundary layer lubricationxe2x80x9d, and this lubrication is less susceptible to working, so it produces considerably less heat, all while protecting the critical surfaces from scoring and the welding of surface finish asperities.
Moreover, a bearing lubricated with high performance grease does not require compressed air and the pumps, filters and dryers identified with it, nor does it require a complex system of ducts to deliver lubrication to critical surfaces in the bearing. Indeed, the machine tool builder simply prelubricates the bearing with a small quantity of the high performance grease.
However, the grease does not achieve its full effectiveness until it is xe2x80x9cbroken inxe2x80x9d, and this demands additional down time for the spindle and the costs associated with it. Typically protective coatings are first removed from the bearing components. Then a small quantity of high performance grease is applied to critical surfaces in the bearing. Next the bearing is operated to better distribute the grease over the critical surfaces and to evacuate unnecessary grease. Basically, the objective of the break-in is to spread the lubricant along the critical surfaces with a thickness that will not support working of the grease. In addition, the operation of the bearing during break-in forces excess grease out of the bearing components. Accordingly, the seals protecting the critical surfaces from external contaminants are installed as the final step in this process. During the break-in, which may consume a full day, the temperature of the bearing is constantly monitored to insure that it does not undergo a rapid rise. Such a rise would generally indicate one of two problems. First, the absence of adequate boundary layer lubrication along the critical surfaces would cause a rise in temperature. Secondly, work is exerted by the components in churning any excess grease, work that results in heat and a corresponding increase in temperature. Also, a spindle bearing will outlast the grease, so to derive the maximum life from the bearings, the bearing must be relubricated from time-to-time. This requires removal of the bearings from the spindle and housing, cleaning the bearings, repacking the bearings with new high performance grease and then reassembling the spindle and housing. And, of course, the new grease must be broken in.
Owing to the cost of repacking spindle bearings with new high performance grease, machine tool operators often delay this maintenance procedure for as long as possible sometimes beyond the point of failure for the grease. And when the grease in a bearing fails, the bearing usually experiences scoring along its critical surfaces.
Also, high speed bearings lubricated with high performance grease require more effective seals than those lubricated with other systems. Standard grease tends to accumulate at each seal and produce a barrier of its own to the ingress of contaminants. Both oil-air and oil mist systems maintain a slight positive pressure within the bearing, and this serves to retard the ingress of contaminants. Circulating oil tends to flush away contaminants, which are removed from the oil at filters. But a bearing lubricated with high performance grease contains very little of it, so it does not accumulate in the form of a barrier, nor does it flush away contaminants or function under a positive pressure.
The present invention resides in the combination of an antifriction bearing and an injector for discharging minute quantities of lubricant into the bearing at selected intervals so that the bearing maintains a film of lubricant over its critical surfaces, but does not have lubricant accumulate within it. The invention also resides in a process for lubricating an antifriction bearing by directing minute quantities of lubricant into it at selected intervals based on feedback from monitored system operating parameters, such as bearing temperature, speed, and running time subsequent to an injection of lubrication. The invention also consists in the parts and in the arrangements and combinations of parts hereinafter described and claimed.